A marine LNG containment system is a liquid tight compartment with thermal insulation. The LNG is kept very cold in the compartment and the purpose of thermal insulation is to insulate the LNG from influx of heat and to protect the hull construction of the LNG ship from the very low temperature of LNG.
Marine LNG containment systems are categorized by the International Maritime Organization (IMO) in the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk, normally known as the IGC Code. The IGC code categorizes present LNG cargo containment systems as membrane and independent tank types A, B and C.
Membrane LNG containment systems are based on a thin LNG barrier (primary barrier) and a collecting barrier (secondary barrier) outside the LNG barrier. Both barriers are supported through thermal insulation to the adjacent ship hull structure. Such LNG containment is not self-supporting in that the ship hull forms the load bearing structure. Membrane LNG containment systems are provided with a secondary barrier to ensure the integrity of the total system in the event of a primary barrier leakage.
Independent tank types A, B and C for liquid gases are completely self-supporting primary barriers which do not form part of the ship hull structure. With self-supporting it is meant that the load from the liquid gas inside the tank is taken up by the tank structure and plating and that the tank is transferring loads to the hull structure through supports.
An IMO independent tank type A is a tank which is designed primarily using classical ship-structural analysis procedure. If the cargo temperature at atmospheric pressure is below −10° C., a complete secondary barrier around a type A tank is required. The secondary barrier is constructed to withstand full collapse of the primary barrier.
IMO independent tank type A has traditionally been used on ships for liquid petroleum gases, also called LPG. The lowest temperature of LPG is −55° C., and on ships for LPG with IMO independent tank type A the hull compartment is made of low temperature steel to collect LPG in case of collapse of the tank (primary barrier). The hull structure is thus acting as the secondary barrier on ships for LPG with IMO independent tank type A. Ship hull acting as a secondary barrier for LNG is not allowed by the IGC Code due to the very low temperature of LNG.
Type B and C tanks are known tank types used for LNG on ships. They differ from Type A tanks in that type B and C tanks are constructed so that full collapse of the tank (primary barrier) shall not be possible, and consequently a full secondary barrier is not required.
Type B and type C LNG tanks are transferring loads to the hull structure through supports and the thermal insulation is attached to the tank (primary barrier). This is similar to Type A tanks for LPG.
The company Conch LNG pioneered the concept of bulk transport of LNG, and
LNG containment systems. The Conch concepts were based on a LNG primary barrier placed on load bearing thermal insulation. The Conch design is presently not used as LNG containment.
In the Conch design, such as described in U.S. Pat. No. 3,974,935 (Conch), the primary LNG tank is supported on spaced apart wooden bearer members (e.g. balsa wood) which are part of the outer insulation. The insulation layer supporting the bottom of the tank including bearer members of load bearing material is in direct contact with the primary barrier and supporting the loaded weight of the tank. Forces are transferred directly from the primary barrier to the secondary barrier, and an impact on the primary barrier may be directly transferred to the secondary barrier, and vice versa.
The present invention differs from the Conch patent in that in the present invention the self-supporting primary barrier and the secondary barrier are separately connected with the hull compartment to prevent transfer of forces between the primary barrier and the secondary barrier.
The present invention also differs from the Conch patent in that in the present invention there is an access space on all sides (above, below and on all sides) between the primary and secondary barrier.
The present invention differs from membrane LNG containments in that in membrane containments the primary barrier is not a rigid self-supporting tank, but a thin membrane that transfers the loads from the LNG, through both the primary and secondary barriers and to the hull structure.
The present invention differs from IMO type B and IMO type C LNG containments in that the present invention has a complete secondary barrier. The present invention differs from IMO type B and IMO type C containments in that in the present invention the LNG tank, defined as primary barrier in the IGC Code, and the thermal insulation, defined as secondary barrier in the IGC Code, have no direct structural connection and do not transfer any loads directly between them. An impact force on one of the barriers will not be transferred to the other barrier, while on type B and C tanks the thermal insulation is attached directly to the tank. The present invention differs from present IMO type A containment for liquefied gas in that on the present IMO type A containments for LPG the supports for the primary barrier are transferring loads to the adjacent secondary barrier structure. The present invention is an arrangement for containment of LNG where the supports for the primary barrier are transferring loads to adjacent structure so arranged that load transfer to secondary barrier is prevented.